Optical data storage systems store and retrieve large quantities of information on a suitably formed, optically accessible medium, such as a rotating "compact disc" (CD) or translating ribbon. The encoded information is usually accessed by focussing a laser beam onto a reflective data layer embedded in the medium and detecting the reflected light beam. In practice, usually both the medium and the light beam are moved to provide access over the entire data surface in a timely manner, yielding an acceptable data access time as well as data transfer rate. Various general types of optical data storage systems have been developed thus far. In a ROM (Read Only Memory) system, data is embedded in the medium at the time of manufacture and is not normally altered by the user. A WORM (Write Once Read Many) system, on the other hand, allows a user to write information into the medium, and thereafter remains unaltered. Such systems are particularly suitable for archives because of their exceptionally high data densities and potentially low cost-per-byte. Erasable optical systems, such as magneto-optic or phase change recording, are also becoming more competitive and may some day supplant conventional magnetic recording as the cornerstone of machine-readable data storage. Unfortunately, due to opto-mechanical constraints and conventional optical data structure, access time in many optical storage systems is not yet competitive with conventional magnetic disc systems. Nevertheless, whereas magnetic data storage systems provide good access time and erasability, optical data storage can provide a unique combination of superior performance features that make them most appropriate for large memory applications. Indeed, despite of their present shortcomings, optical information storage systems promise to offer low cost-per-byte, improved access characteristics and unparalleled storage density.
With the explosive growth of information technology, the horizons of information processing and data storage applications are expanding rapidly. Yet, full motion high resolution video/audio and document image processing are not practical with current CD-ROM technology. Double-, triple- and quadruple-density/speed CD-ROM may be readily available in the near future. However, the data transfer rates necessary for full-motion video tax the current and near future CD-ROM capabilities for providing such high profile items as full-length feature films. So-called information super highways may augment CD-ROMs, but these high speed networks will require new higher capacity drives that can locally store and transfer information. Other examples, such as digital cameras or 3-D image displays, will undoubtedly create additional demands on high capacity data storage technology. Moreover, considering the current growing popularity of optical CDs and CD-ROMs, backward compatibility of any new storage technology is highly desirable, thus placing additional constraints on design and economic considerations. With the emergence of the information age and associated data intensive applications, there is clearly a demand for higher density information storage capabilities.
In most commercial optical storage systems, such as the CD-ROM, digital data is decoded by virtue of an optical intensity modulation reflected from a data layer embedded in the storage medium. A noteworthy exception is that of magneto-optical storage media wherein a magnetic material induces a change in light polarization rather than light intensity. Still in others, optical phase may be modulated by local changes in refractive index, which however, ultimately gives rise to intensity changes during the read process. The signal:noise ratio in commercial CD-ROMs is sufficient to provide bit error rates compatible with the computer industry, typically better than about 10.sup.-12. With current CD-ROM storage capacity at about 1 GB and transfer rates at about 1 MBps, CD-ROM has enjoyed a popular growing consumer base oriented to multimedia and other data intensive applications. Likewise, maintaining consumer interest mandates increased storage capabilities for anticipated new applications.
It is generally acknowledged that increasing the areal storage capacity of conventional optical CDs demands increasing the areal density of the pits and lands that comprise a conventional data layer. In addition, increasing the areal data density of an optical CD will also entail changes in optical and tracking hardware. For example, a shorter wavelength laser light source is generally recognized as a high leverage technique for increasing storage capacity. Indeed, in most conventional optical storage arrangements, the light source wavelength sets the fundamental limit for areal packing density. Unfortunately, efforts to produce a cost-effective blue laser have proven time-consuming and costly, with limited success thus far. Factoring in all-near term improvements to conventional CD-ROM technology, a single-layer areal density may be expected to increase by only a factor of 4.times. to 10.times..
Considering briefly the optical interactions within a storage medium, typically electromagnetic radiation is scattered or re-radiated by matter at the same wavelength as the exciting radiation. Such common macroscopic phenomena as reflection and refraction of light are prime examples of "elastic" light scattering. CD-ROMs utilize these effects in generating the optical signal. However, when a material absorbs part of the incident electromagnetic energy, it may be possible for it to emit radiation at another wavelength, comprising a so-called "inelastic" light scattering process. Radiation-induced electronic, vibration or rotational transitions in molecules comprise a large portion of the commonly observed inelastic light scattering processes. At least in principle, the ability to analyze and control the optical emission spectrum of a selected material provides additional means by which to optically encode information. Until recently however, little commercial progress has been made in storage media which exploit potential multi-wavelength optical interactions. Such processes may for example include spectral hole-burning, fluorescence or nonlinear optical interactions. Nevertheless, because of the potential massive increase in storage capacity, so-called frequency domain or spectrally based storage mechanisms have continued to attract considerable attention.
In 1928, the Indian Physicist C. V. Raman discovered a new optical interaction characterized in that the wavelength of a small fraction of the radiation scattered by certain molecules differs from that of the incident beam. Furthermore, the shifts in wavelength depend upon the detailed physical and chemical composition of the molecules responsible for the scattering. He was subsequently awarded the 1930 Nobel prize in physics for this discovery and his systematic exploration of it. With the advent of lasers, Raman scattering, as it has come to be called, has evolved into a powerful analytical tool used by physicists, chemists and clinicians worldwide.
Today Raman spectroscopy is typically performed by irradiating a sample with a powerful laser source of visible or infrared monochromatic radiation. During irradiation, the spectrum of the scattered radiation is measured with a suitable spectrometer. At best, Raman intensities are: about 10.sup.-6 or less of the source intensity. Consequently, their detection and measurement is difficult and has received much attention from various research communities over the years. Resonance Raman Spectroscopy (RRS) significantly enhances the Raman efficiency by essentially tuning the exciting radiation to an electronic absorption peak of the Raman active material. Another relatively recent and useful technique comprises obtaining enhanced Raman spectra of materials which are deposited on colloidal metal particles or rough metal surfaces, particularly silver, gold or copper surfaces. Appropriately dubbed "Surface-Enhanced Raman Scattering" (SERS), such effects, although not fully understood, can yield Raman enhancements from 10.sup.3 to 10.sup.6. The combination of RRS and SERS techniques have rendered Raman spectroscopy an invaluable tool in clinical and biophysical research.
As a so-called second order optical process, historically Raman scattering was believed to be suitable for primarily analytical purposes. Recently, however, Surface Enhanced Raman Scattering (SERS) has been proposed for use as a means for optical data storage in a paper by T. Vo-Dinh and D. L. Stokes entitled "Surface-Enhanced Roman Optical Data Storage: A New Optical Memory With Three-Dimensional Data Storage", Rev. Sci. Instrum., Vol. 65(12), p. 3766, 1994. U.S. Pat. No. 4,999,810 by T. Vo-Dinh, herein incorporated by reference, discloses the use of a SERS active media for the purpose of optical data storage. As disclosed by Vo-Dinh, a Raman active medium is deposited on a SERS-promoting substrate in a manner which yields enhanced Raman light emission of the medium when irradiated with light of the appropriate wavelength. Preferably, such a process and sensing means should be efficient enough to yield a signal:noise ratio acceptable for data storage and transfer applications. In a proposed write procedure, reminiscent of other optical recording systems, the microenvironment of the SERS medium may be altered in accordance with the data to be stored. In a subsequent read process, the emission or absorption characteristics of the medium will be modulated and detected in accordance with stored data. U.S. Pat. No. 5,325,342 by Tuan Vo-Dinh, also herein incorporated by reference, discloses the application of SERS active media for erasable optical data storage. This patent further discloses the potential for increased data storage density by exploiting multiple SERS layers, in addition to incorporating optical near-field techniques. Despite the acknowledged potential for high capacity optical storage, there is no specific mention of how the spectral characteristics of SERS-based medium would benefit optical data storage. Indeed, it is well known that in spite of surface enhancement contributions to Raman scattering, the total relative scattered energy is still very small compared to that of commercial optical data storage systems.